One type of power supply system includes a high frequency inverter having DC power supplied to it by a rectifier, in turn driven by an AC power line source. The inverter includes an output transformer having plural secondary windings, each of which drives a separate load. One of the loads is a main load while the other loads normally draw considerably less current than the main load and are denominated as auxiliary loads. Typically, the load connected to a secondary winding around which a control loop is closed can be considered as the main channel load and the circuitry supplying such a load with current is considered as a main channel.
If the main load current is less than a predetermined value the high frequency inverter does not operate in a manner to enable the auxiliary loads to be regulated. Generally, none of the auxiliary loads draws enough current to enable the inverter to operate in such a manner as to achieve auxiliary load regulation. Proper output regulation of all loads is thus not attained unless the inverter is supplying at least a minimum amount of power to the output transformer thereof. Hence, it is necessary for the main load connected to the secondary windings to draw enough current to cause the inverter to supply at least the predetermined minimum power to the output transformer.
In the past inverters have been maintained at or above the minimum output power thereof required for proper operation by sensing the main channel current and controlling an impedance coupled to the main channel. The prior art devices have typically used bulky current shunts, power resistors or large non-saturating transformers. These prior art devices are beset by problems associated with volume, weight and cost. In addition, the prior art structures cannot handle a wide current range of the main channel without sacrificing sense resolution and/or power dissipation.
It is, accordingly, an object of the present invention to provide a new and improved circuit for controlling the impedance coupled to an inverter of the type which must supply a minimum amount of power to an output transformer for proper operation.
Another object of the invention is to provide a new and improved circuit for monitoring main channel current of an inverter and for causing an impedance in the main channel to vary to maintain the main channel current above a predetermined minimum value required for proper regulation of other channel outputs.
Another object of the invention is to provide a new and improved inverter for driving a main channel output having a current sensor which controls the impedance of the main channel so that the controlled impedance of the main channel always has a relatively high, non-dissipating value except when the main channel current drops below a predetermined minimum value necessary to sustain proper regulator operation.
An additional object of the invention is to provide a new and improved inverter for driving a main channel load wherein a current sensor and impedance for the main channel are arranged so that, in response to the main channel current dropping below a minimum current necessary for proper regulator operation, the impedance controlled by the current sensor varies by an amount just necessary to provide proper compensation for the insufficiency of the main channel current.
A further object of the invention is to provide a new and improved inverter having main channel current sensing and control circuitry with minimum volume and weight requirements, as well as high sense resolution and low power dissipation.